The present invention relates to a method for manufacturing a bending transducer, a micro pump and a micro valve, and to a micro pump and a micro valve comprising the bending transducer manufactured according to the method.
According to conventional technology a large number of different micro membrane pumps exist, the drive concepts used being electromagnetic, thermal and piezoelectric driving principles. However, almost all of the micro membrane pumps available on the market are driven by piezoelectric driving principles.
The compression ratio of micro pumps is an important parameter that defines the bubble tolerance and the counter pressure capability of the micro pumps when gasses are the media to be pumped. As also for fluid pumps gas bubbles may at any time enter into the pump chamber, the counter pressure capability for liquid pumps is also defined—in practice—by the compression ratio (besides large actuation diaphragm force and low valve leakages). The compression ratio is defined as the ratio between the volume displaced by the pump membrane in one blow or cycle, the so-called stroke volume, and the dead volume, i.e. the minimum volume remaining when the pump membrane has been moved to pump the media contained in the pump chamber out of the pump chamber. The dead volume can also be referred to as the difference in volume between the maximum pump chamber volume and the stroke volume. The compression ratio of known micro pumps is relatively small and within a range of 0.1 to 1.
Patent application publication EP 0 424 087 A1 describes, for example, a micro pump with a piezo-electric means being deformable by voltage signals in first and second directions, i.e. upward and downward, so as respectively to draw liquid into and expel liquid from the liquid reservoir of the micro pump. The micro pumps described in EP 0 424 087 A1, however, are disadvantageous in that they either comprise considerably large dead volumes or only allow small strokes in the upward direction and thus only small stroke volumes.
Furthermore, the compression ratio of piezo-electrically driven micro membrane pumps is typically defined by the following boundary conditions. When applying a positive voltage to a piezo-membrane transducer, the membrane transducer can only be deflected in downward direction. The deflection upwards is only possible by applying a negative voltage, wherein only about 20% of the downward stroke is achievable, as otherwise the piezo ceramics would be depolarized. When restricting the movement of the membrane to the downward movement it is difficult to reduce the dead volume and to increase the compression ratio. Therefore, for conventional micro pumps, see for example the micro membrane pump of US 2005/0123420 A1 and the peristaltic micro pump of U.S. Pat. No. 6,261,066 B1, the piezo-electric means is only moved in one direction and/or the pump chamber is formed such that its contour is adapted to the bend line of the membrane to reduce the dead volume and to thus maximize the compression ratio. This adaptation to the bend line of the pumping membrane is complex and costly with regard to production engineering, and furthermore a complete adaptation to the bend line is typically not possible because the pumping membrane deflects itself not completely symmetrically, for example, because of distortions of the pumping membrane due to the gluing process, so that gaps remain within the pump chamber that reduce the compression ratio. Furthermore, the stroke volume of a standard actuator is limited by the boundary conditions, if the edges of the diaphragm are clamped. Finally, with silicon this alignment can only partly be achieved by etching several steps into the wafer, which causes a high effort.
U.S. Pat. No. 5,759,014 describes a micro pump with a silicon pumping membrane arranged on a glass base plate and an inlet valve and an outlet valve arranged opposite to each other on opposite sides of the pumping membrane. The pumping membrane has an upwardly bulging shape in the rest position. An piezoelectric element is secured to the top of the membrane. In case the piezoelectric element is activated, the membrane displaces downwardly. The bulged shape of the membrane can be obtained by placing under vacuum the chamber located above the hermetically closed membrane or by applying to its upper surface an oxide layer including a pre-stress suitable deformation. Micro pumps according to U.S. Pat. No. 5,759,014 are disadvantageous in that the dead volume caused by the connection spaces between the pumping chamber and the inlet and outlet valve is still considerably high, the achievable bulging heights of the membrane are limited (thus, only facilitating limited compression ratios), and involve a considerable amount of circular regions of silicon oxide on the lower surface of the membrane to prevent sticking or suction of the membrane. In addition, the lateral arrangement of the valves increases the flow resistance in the pump chamber significantly, with that the stroke volume can only transported at very low pump frequencies, limiting the maximal pump rate. Another disadvantage of the buckling by oxide layer or vacuum is the fact, that if a piezo is actuated by a positive voltage, the diaphragm cannot be moved to a completely flat position. Thus, a dead volume at the border of the diaphragm remains.
US2009/0158923 A1 describes a pre-stressing of the pump diaphragm realized by laser welding of two metal layers. This application states that (obviously due to the thermal impact of the welding process) a pre-stressing of the diaphragm and the pump chamber can be realized. However, again, a large dead volume (which is even larger than the dead volume due to the oxide layer in U.S. Pat. No. 5,759,014) remains at the border of the diaphragm after actuation of the piezo.
In fact, the joining of the actuation membrane with the pump chamber by laser welding as depicted in FIG. 8 causes an unavoidable buckling of the membrane, which is not optimized with respect to a minimized dead volume. FIG. 8 shows two schematic drawings of a micro pump with a pump body 810 and a membrane 820 bonded to the pump body by laser welding at the border of the pump membrane. The top part of FIG. 8 shows the pump membrane 820 in a pre-stressed non-actuated state, and the lower part of FIG. 8 shows the same membrane 820 bent down by a piezo element 830 arranged on top of the pump membrane. As can be seen from the lower part of FIG. 8, the membrane 820 is not completely flat, but shows bulges or deflections 840 at the border of the pump membrane that cause an increased dead volume due to the volumes defined by these bulges 840 at the border of the membrane.
US 2004/0036047 A1 and US 2006/0027772 A1 describe normally closed valves. Formed by a stack of two silicon chips, wherein the lower silicon chip comprises the inlet and outlet of the valve, and wherein the upper chip mounted on the lower chip, comprises a valve chamber recess, a valve lip and a tappet on a side facing towards the lower chip, and a recess on the opposite side of the upper chip facing away from the lower chip for defining a membrane, wherein on the membrane above the tappet a piezo drive is arranged to move a valve shutter formed in the lower chip down to open the valve. In a closed state, i.e. when the piezo ceramic is not actuated, the valve lip fluidly decouples the valve inlet from the valve chamber. In case the piezo ceramic is activated, the piezo ceramic moves down the valve shutter that is connected to the membrane via the tappet. In this case the valve lip does not abut on the tappet anymore and the valve is open. It has been recognized that a membrane sometimes tends to be deflected in the downward direction after the production of the valve. In case these deflections in a downward direction are to large, the valve might not fulfill the denseness requirements for normally closed valves or might open at slight pressures in reverse direction to the membrane. Such undesired flows in the non-actuated state of the valve need are disadvantageous and can be even critical in the fields of medical technology or fuel cells.
It is the object of the present invention to provide a method of manufacturing a bending transducer allowing to eliminate one or all of the above mentioned disadvantages of conventional technology. It is a further object of the present invention to provide a micro pump that is capable of providing high compression ratios and that can be easily production engineered. An even further object of the present invention is to provide a micro valve with reliable denseness characteristics.